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Parity-Violating Electron Scattering

Parity-Violating Electron Scattering. Jeff Martin University of Winnipeg. Parity-Violating Elastic Scattering of Electrons from Protons. Two applications we will study tonight: Strange quark structure of the nucleon. Tests of standard electroweak theory. g. p. e.

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Parity-Violating Electron Scattering

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  1. Parity-Violating Electron Scattering Jeff Martin University of Winnipeg

  2. Parity-Violating Elastic Scattering of Electrons from Protons • Two applications we will study tonight: • Strange quark structure of the nucleon. • Tests of standard electroweak theory.

  3. g p e ElectromagneticElastic Electron Scattering • Scattering cross-section depends on two “form factors” GE(Q2), GM(Q2). • At small Q2, form factors are Fourier transforms of spatial distributions of charge and magnetization densities in the proton. k’ q = k – k’ “4-momentum transfer” k A useful variable:

  4. Relationship to Quarks • The charge and magnetization are carried by quarks • We can do the same experiment for the neutron (udd) isospin symmetry

  5. Z0 p e The Extra Handle:Z0 scattering

  6. Parity Violating Asymmetry forward ep backward ep backward ed kinematical factors Note: Asymmetry is of order ppm

  7. The Proton’s Weak Charge As Q2  0 MNC MEM measures Qp – proton’s electric charge measures Qpweak– proton’s weak charge At tree level in the standard model: A sensitive, low-energy extraction of the weak mixing angle.

  8. Physics: The Running of sin2W 12 GeV QW(e) present: “d-quark dominated” : Cesium APV (QAW): SM running verified at ~ 4 level “pure lepton”: SLAC E158 (QeW ): SM running verified at ~ 6 level future: “u-quark dominated” : Qweak (QpW): projected to test SM running at ~ 10 level “pure lepton”:12 GeV e2ePV (QeW ): projected to test SM running at ~ 25  level

  9. Qpweak & Qeweak – Complementary Diagnostics for New Physics JLab Qweak SLAC E158 (complete) - (proposed) (published) ±0.006 • Qweak measurement will provide a stringent stand alone constraint • on Lepto-quark based extensions to the SM. • Qpweak (semi-leptonic) and Moller (pure leptonic) together make a • powerful program to search for and identify new physics. Erler, Kurylov, Ramsey-Musolf, PRD 68, 016006 (2003)

  10. Summary of PV Electron Scattering Experiments publishing, running publishing, running x2, published x2, running publishing, running 2008 K. Kumar DHB, 17 June 2005

  11. G0 Forward-AngleMeasurements Det 8 pions elastic protons inelastic protons • Elastic proton detection • toroidal focusing spectrometer • Time-of-flight distinguishes pions and protons

  12. G0 Forward-Angle Configuration at Jefferson Lab superconducting magnet (SMS) cryogenic supply G0 beam monitoring girder detectors (Ferris wheel) target service module Beam

  13. Largest Systematic Effect: Backgrounds detector 8 • Determined using fitting techniques • Large asymmetry from hyperon production, decay, rescattering

  14. G0 forward-angle experiment – final results GEs+GMs, Q2 = 0.12-1.0 GeV2 2 test taking into account random and correlated errors: the non-vector-strangeness hypothesis is disfavored at 89%

  15. Comparison to World Data Q2=0.23 GeV2 95.5% CL Q2=0.1 GeV2 Q2=0.48 GeV2

  16. Empirical Fit: GEs and GMs Separately • Compare GEs with GEn, and GMs with GMp -1/3s/p = -18% -1/3GEs(0.2)/GEn(0.2)~40%

  17. Upcoming Data-Taking:The year of G0 In coming years, G0 will run at backward angles in order to truly separate the electric and magnetic form factors. • March 15 – April 29, 2006: Q2 = 0.6 GeV2. • July 21-Sept. 1, 2006: Q2 = 0.23 GeV2. • Sept. 22-Dec. 22 2006: Q2 = 0.6 GeV2. • 2007: finish low Q2.

  18. Backward-Angle Measurements • Electron detection (Note: VERY different systematics) • Add Cryostat Exit Detectors (“CED’s”) to define electron trajectory • Add aerogel Čerenkov counter to reject - • Measurements on H and D to separate GMs, GAe FPD #16 CED #9 Čerenkov magnet elastic e- inelastic e- or photo - FPD #1 CED #1 beam target

  19. Recent progress: • - Target installed • Beamline/Shielding in progress • Upstream Girder in progress • - Cosmics testing ongoing

  20. theory: Lewis, Wilcox, Woloshyn DGE DGM s s DGA = 0.22 e = 0.032 = 0.13 G0 contribution 2007-8 • Very soon – high precision data from Happex at 0.1 GeV2

  21. Elastically Scattered Electron Luminosity Monitors Region III Drift Chambers and Quartz Scanner Toroidal Magnet Region II Drift Chambers Eight Fused Silica (quartz) Čerenkov Detectors Region I GEM Detectors Collimator with 8 openings θ= 8° ± 2° electronics 35cm Liquid Hydrogen Target Polarized Electron Beam

  22. QpWeak Toroidal Magnet - QTOR scattered e envelope • 8 toroidal coils, 4.5m long along beam • Resistive, similar to BLAST magnet • Pb shielding between coils • Coil holders & frame all Al • Bdl ~ 0.7 T-m • bends elastic electrons ~ 10o • current ~ 9500 A beam

  23. Pb pre-rad scattered beam quartz PMT 2” air-core light guide Č Quartz Scanner Detector • Scans in 2D through scattered beam near the main Quartz detector for a variety of purposes: • Fiducialization and “light map” of main detector • backgrounds (inelastics) • confirm linearity of main detector response with beam current • Q2 determination • Similar technique used in both E158 and HAPPEx • UWinnipeg RTI proposal to NSERC submitted Oct. 2005.

  24. Qweak status • Magnet assembly and verification beginning. • Main detectors under construction at JLab. • Tracking chamber development underway by US university groups. • Target development underway. • Parasitic beam tests of some instruments conducted simultaneously with G0 • First run 2008-2010: 8% → 4% • More running 2010-2012: 4% → 2.5%

  25. Summary • PV electron scattering is a useful tool for: • strangeness form factor determination. • extraction of sin2W for standard model test. • G0 Forward angle results published. • G0 Backward angle running 2006-7. • Qweak beginning in 2008.

  26. Summary of Systematic Effects

  27. Anticipated QpWeak Uncertainties  Aphys/AphysQpweak/Qpweak Statistical (2200 hours production) 1.8% 2.9% Systematic: Hadronic structure uncertainties -- 1.9% Beam polarimetry 1.0% 1.6% Absolute Q2 determination 0.5% 1.1% Backgrounds 0.5% 0.8% Helicity-correlated Beam Properties 0.5% 0.8% _________________________________________________________ Total 2.2% 4.1% 4% uncertainty on QpW→ 0.3% precision on sin2W at Q2 ~ 0.03 GeV2 (Erler, Kurylov, Ramsey-Musolf, PRD 68, 016006 (2003)) QpW = 0.0716  0.0006, theoretical extrapolation from Z-pole 0.8% error comes from QCD uncertainties in box graphs, etc.

  28. G0 Backward Angle:Parasitic Physics • Axial structure of the nucleon and the anapole moment. • Parity-violation in electro and photo excitation of the Delta resonance (inelastic electron and photopion asymmetries). • Beam normal asymmetries and two-photon exchange for form factor systematics (theory: Blunden et al).

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